In general, most of internal combustion engines, such as diesel engines, are configured as a multi-cylinder engine provided with a plurality of cylinders. In a case where an OHC (overhead cam carrier) is employed for a valve mechanism in the multi-cylinder engine, multi-cylinder engine includes a camshaft having a plurality of cams for opening and closing an intake valve and an exhaust valve arranged on an upper portion of the inside of each cylinder, and the camshaft is rotatably supported by a cam carrier provided on the top of a cylinder head.
In the multi-cylinder OHC engine, the cam carrier includes a pair of longitudinal frames provided parallel to an axial direction of the camshaft and a plurality of transversal frames connected to the pair of longitudinal frames to be spaced from each other. The transversal frames are provided to correspond to the number of cylinders so as to be positioned between each of the cylinders of the engine. Also, cam bearings arranged on the respective transversal frames support the camshaft.
Further, since a coaxiality accuracy in machining cam bearing positions with respect to the respective cylinders has a great influence on reliability and durability, the longitudinal frames and the transversal frames are monolithic with each other and then coaxial machining is performed on such a monolithic structure, so that the cams on the camshafts can be arranged at correct positions. The monolithic cam carrier is fixed on the cylinder head by bolts.
However, if an internal combustion engine, in which the monolithic cam carrier is fixed on the top of a cylinder head, is under a running condition of the engine, under which a difference in temperature between the cam carrier and the cylinder head occurs, such as a running condition in which the engine is suddenly transmitted from a low load running such as a cold state to a high load running, or a running condition in which the engine is suddenly transmitted from the high load running to the low load running or a no-load running (a state where an engine brake is activated), there are problems that a relative displacement between the cylinder head and the cam carrier occurs due to a difference in temperature therebetween or that a thermal deformation occurs due to a difference in thermal expansion between a contact surface of the cam carrier with the cylinder head and an upper surface side of the cam carrier opposite thereto.
That is, when the engine running state is suddenly changed, an amount of heat emitted from the cylinders of the engine greatly changes. Therefore, a difference in temperature between the cylinder head and the cam carrier occurs and hence a relative displacement therebetween occurs due to a difference in thermal expansion caused by the difference in temperature. As a result, fretting which is a surface damage caused when minute reciprocating sliding repeatedly acts thereon occurs.
A contact surface side of the cam carrier which is contact with the cylinder head tends to follow the temperature of the cylinder head. An upper surface side of the cam carrier which is opposite to the cylinder head is difficult to follow the temperature of the cylinder head since a heat transfer thereto is slower and an amount of heat radiated therefrom is large. Therefore, a difference temperature occurs between the contact surface side and the upper surface side and a difference between an amount of thermal expansion of the contact surface side and an amount of thermal expansion of the upper surface side occurs, thereby causing a thermal deformation inside the cam carrier. If the thermal deformation is caused, a position or inclined angle of the cam bearings relative to the camshaft is changed, thereby causing problems, such as wear or seizing of the cam bearings.
That is, in a cylinder head structure 1X for an internal combustion engine according to the related art as shown in FIG. 6, if a cylinder head 10 is further thermally expanded (ΔLb) as a temperature thereof is suddenly increased from a normal state where a temperature of a cam carrier 20 is substantially even, a contact surface 20r side of the cam carrier 20 has a relatively large elongation amount ΔLr by following a thermal expansion of the cylinder head 10, and hence a position Pr of a bolt 50, which is screwed in the cylinder head 10, on the contact surface 20r side is also displaced. On the other hand, an upper surface 20t side, from which a large amount of heat is radiated, cannot follow the increased temperature and hence has a relatively small elongation amount ΔLt. Therefore, a position Pt of the bolt 50 on the upper surface 20t side is offset from the position Pr on the contact surface 20r side. That is, the cam carrier 20 is thermally deformed due to a difference in thermal expansion therein. Meanwhile, FIG. 7 is a view schematically drawn focusing on elongation only in a lateral direction of FIG. 7 for the purpose of explanation, and thus the overall elongation, such as in a upward and downward direction, is not exactly shown therein. Also, when the temperature is increased, the relation “ΔLb≥Lr≥Lt” is obtained, whereas when the temperature is decreased, the relation “ΔLb≤Lr≤Lt” is obtained.
In this context, in order to prevent abnormal noises from being generated by an expansion of a camshaft pitch between an intake camshaft journal portion and an exhaust camshaft journal portion and an increment of a valve clearance, a cylinder head has been proposed, in which a cam carrier, which is separate from an aluminum cylinder head main body, is placed on the cylinder head main body, the cam carrier is fabricated by aluminum die-casting, and then cast iron bearing member is casted in the cam carrier, thereby reducing an amount of thermal deformation of the cam carrier (e.g., see Patent Document 1).
However, in this cylinder head, since the cast iron bearing member is casted in the aluminum cam carrier, there arises a problem that due to a difference in thermal expansion between aluminum alloy and cast iron, a thermal stress caused by a difference in amount of thermal deformation always acts outside a specific temperature range. In addition, since the amount of thermal expansion of the cam carrier is suppressed to be small, there arises a problem that a relative displacement between the cylinder head main body and the cam carrier placed thereon is increased.